Weight loss is a frequent concomitant observation in dementia syndromes and is known to worsen the prognosis of elderly patients. This is a retrospective cross-sectional study of 1575 consecutive geriatric patients to obtain data about body weight and body composition in relation to gender and cognitive function. Fat mass (FM) and fat-free mass (FFM) were determined by bioelectric impedance analysis. Subjects with severe cognitive dysfunction (MMSE<11) had a significant lower body weight (6.5%), BMI (5.7%), FM (15.7%) and fat mass index (14.3%) than subjects without cognitive dysfunction (MMSE>26). FFM was not significantly decreased (2.1%). Subgroup analysis showed that mean body weight is closely related to the degree of cognitive dysfunction. Gender-related analysis showed no significant difference in body weight, BMI, FM and fat-mass index (FMI) between male subjects with severe cognitive dysfunction and male subjects with normal cognitive function. Only FFM was significantly decreased (7.0%) in males with severe cognitive dysfunction. Findings of this study indicate that patients with cognitive dysfunction lose substantial amounts of body weight, related to the degree of cognitive dysfunction. In this connection, female subjects seem to lose more weight than male subjects. At the same time female subjects predominantly lose FM, whereas male subjects seem to lose mainly FFM. Therefore patients with cognitive dysfunction should be regularly screened for weight loss and malnutrition to enable early nutritional intervention to prevent relevant weight loss. Future studies on weight loss in dementia should consider gender-related differences in body composition and weight loss.